1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device.
2. Description of the Related Art
A solid state image sensor is one of typical semiconductor devices which have a strict leakage current requirement. With the advance of solid state image sensors, high-quality, inexpensive digital cameras and digital video cameras have become very popular. Each solid state image sensor includes a pixel area and a peripheral area. The pixel area includes a plurality of pixels. Each pixel includes, for example, a photoelectric converter (photodiode), a transfer transistor, and an amplification transistor. The amplification transistor may sometimes be shared by a plurality of pixels. The peripheral area includes a MOS transistor. The peripheral area processes a signal output from each pixel of the pixel area and controls the pixels in the pixel area. The circuits in the pixel area and the peripheral area can be manufactured by the common steps in the CMOS manufacturing process.
A semiconductor compound layer (silicide) of a high melting point metal is selectively formed on the source, drain, and gate electrodes of the MOS transistor of the peripheral area, thereby increasing the operation speed of the MOS transistor. On the other hand, forming a silicide layer on the photoelectric converter of the pixel area increases the leakage current of the photoelectric converter, thereby degrading the photoelectric conversion characteristics.
Japanese Patent Laid-Open No. 2011-29604 discloses suppression of the leakage current of a photoelectric converter by forming a silicide layer only in the peripheral area out of the pixel area and peripheral area. Japanese Patent Laid-Open No. 2007-165864 discloses a method of forming an interlayer insulating film on a silicon nitride film formed on a silicon oxide film and forming a contact hole extending through the silicon oxide film, the silicon nitride film, and the interlayer insulating film. According to this method, an opening is formed in the interlayer insulating film using the silicon nitride film as an etching stopper, and the silicon nitride film and the silicon oxide film are etched through the opening.
As disclosed in Japanese Patent Laid-Open No. 2007-165864, the method of forming the silicon oxide film on a semiconductor substrate, forming the silicon nitride film on the silicon oxide film, forming the interlayer insulating film (silicon oxide film) on the silicon nitride film, and then forming the contact hole is known. When continuously etching the silicon nitride film and the silicon oxide film under the silicon nitride film through the opening formed in the interlayer insulating film under the same conditions, an etching selectivity to the semiconductor substrate cannot be kept sufficiently high. That is, etching conditions suitable for both the silicon nitride film and the silicon oxide film are insufficient for the etching selectivity to the semiconductor substrate. For this reason, the surface of the semiconductor substrate may be excessively etched. This may cause degradation of the performance of semiconductor devices, such as an increase in leakage current. The silicon nitride film and the silicon oxide film under the silicon nitride film may be etched under different etching conditions to increase the etching selectivity of the silicon oxide film to the semiconductor substrate. However, in this case, it takes time to change the etching conditions, thereby decreasing the throughput.